1. Field of the Invention
The invention relates to a process for the preparation of a catalyst suitable for use in the isomerization of normal paraffins.
2. Description of the Prior Art
Catalysts that have been prepared by depositing one or more noble metals of group VIII on mordenite have a high activity and selectivity for the isomerization of normal paraffins. For the catalysts to be suitable for this purpose the exchangeable metal cations originally present in the mordenite should be replaced almost completely by hydrogen ions. This replacement may be carried out by treating the mordenite with an aqueous solution of an acid or by treating the mordenite with an aqueous solution of an ammonium compound and subsequently calcining it. See, e.g., U.S. Pat. No. 3,301,917.
As taught in U.S. Pat. No. 3,442,794, catalysts prepared by depositing one or more noble metals of group VIII on mordenite, which mordenite had previously been treated with an aqueous solution of an acid and subsequently with an aqueous solution of an ammonium compound, have a greater activity in the isomerization of normal paraffins than catalysts prepared by depositing one or more noble metals of group VIII on mordenite, which mordenite had been treated either with an aqueous solution of an acid or with an aqueous solution of an ammonium compound. Futher, as taught in U.S. Pat. No. 3,691,009 and U.K. Patent application No. 277/74 filed Jan. 3, 1974, in the isomerization of normal paraffins the activity of the catalysts prepared by depositing one or more noble metals of group VIII on mordenite that had been subjected to the abovementioned two-step treatment, could still be improved considerably by carrying out the acid treatment of the mordenite with a solution prepared by incorporating potassium and/or sodium ions in an aqueous acid solution with a normality of at least 0.5 and at most 3.0, in an amount which, expressed in grams per liter (g/l), is at least ten times the normality of the aqueous acid solution.
From the catalyst base material obtained in the way described above, catalyst particles can be prepared by shaping under high pressure. In order to give these catalyst particles sufficient crushing strength for use on a technical scale, it is common practice to mix the catalyst base material thoroughly with an amorphous binder material consisting substantially of one or more oxides of metals of the groups II, III and IV, before subjecting it to shaping under high pressure.
It had been found in the past that the improved activity for the isomerization of normal paraffins, which improvement had been obtained by replacing the classic one-step treatment of the mordenite by the two-step treatment, using an aqueous acid solution in which no alkali metal ions had been incorporated, is fully retained when the catalyst base material is thoroughly mixed with an amorphous binder material, before shaping under high pressure. Therefore, it was assumed that this retention of activity would apply also to the additional improvement obtained by incorporating alkali metal ions in the aqueous acid solution. This assumption was found to be incorrect. It was found experimentally that, if the mordenite present in the catalyst base material had been subjected to the two-step treatment, using an aqueous acid solution in which alkali metal ions had been incorporated, the additional improvement obtained by incorporating alkali metal ions in the aqueous acid solution was completely lost when the catalyst base material was mixed with an amorphous binder material, before shaping under high pressure. What is needed is some process change wherein the improved activity of the alkali metal-modified catalyst is retained after mixing with an amorphous binder material.